JP2008119947A - Multilayer shrink film - Google Patents

Abstract

The present invention provides a multilayer shrink film that exhibits high shrinkage at about 110 ° C. or higher, low shrinkage at about 100 ° C. or lower, and can be sealed at a low temperature, but the films are not fused together by heating in a microwave oven.
There are at least three layers of both surface layers and an inner layer sandwiched therebetween, and the inner layer has a molecular weight distribution (Mw / Mn) of 4 to 10 and a density of 0.900 to 0.930 g / cm. ³ , an ethylene-α-olefin copolymer having a melt tension of 25 to 100 mN at 240 ° C. and a melting main peak temperature of 118 ° C. or less (at least one selected from ethylene and α-olefins having 3 to 18 carbon atoms) A multilayer shrink film containing 70 to 100% by weight of a copolymer with a monomer and having a melt tension at 240 ° C. of the resin composition constituting the inner layer of 25 to 100 mN.
[Selection figure] None

Description

The present invention relates to a multilayer shrink film that is tightly attached to a packaged object by heat shrinkage.
In particular, the present invention relates to a multilayer shrink film suitable for packaging an object to be packaged that is reheated in a microwave oven after packaging a lunch box container or a side dish container.

  Conventionally, shrink film for packaging can quickly and tightly package multiple products at the same time, regardless of the shape and size of the package, and the resulting package has a beautiful appearance and exhibits a display effect. In addition, it is often used for packaging food, sundries, etc. because it increases the value of the product, keeps the contents hygienic, and facilitates visual quality confirmation.

A typical example of a packaging method using such a shrink film for packaging is a method in which the film is heat-shrinked with hot air or the like after the contents are primarily packaged with a little allowance for the film, for example, pillow shrink packaging. In this method, generally, a food or other packaged object stored in a container or tray is covered with a film in a cylindrical shape, and then a sealing wire is formed on the back surface of the packaged object by a center seal device such as a rotary roller type. And then heat-sealing both open ends of the tubular film to form a bag, heat treatment with hot air in a box called a shrink tunnel, This is heated and shrunk while air is deaerated. In addition to the above, this pillow shrink packaging includes a three-sided seal and a method of heating a four-side sealed bag-like film.
Main examples of the package to be subjected to such pillow shrink packaging include bento and side dishes using heat-resistant containers such as polystyrene with a lid and PP with filler, foam polystyrene without a lid, PP, and paper In any case, a beautiful package with little corner residue can be obtained by wrapping containers and trays with a margin and then shrinking them by blowing hot air.

By the way, in recent years, it is desired to reduce the temperature of shrink wrapping in order to reduce carbon dioxide emissions and to reduce costs by increasing the speed of the packaging line. Better to do.
In particular, from the viewpoint of packaging finish and the like, it is better to have high shrinkage at a temperature of about 110 ° C. or higher used for heat shrink packaging.

  On the other hand, in packaging of a packaged object such as the above-mentioned lunch box or side dish, reheating in a microwave oven is often performed after packaging, and when heated to the highest temperature, the temperature reaches 100 ° C., and the container becomes soft due to heat. . Therefore, when a package packaged using a heat-shrinkable film is reheated in a microwave oven, the container may be deformed due to the shrinkage of the film. In particular, since containers and trays are becoming thinner from the viewpoint of resource saving, such container deformation is likely to occur. Therefore, from the viewpoint of preventing container deformation, the shrinkage characteristics of the packaging film are preferably low shrinkage at 100 ° C. or lower.

  Therefore, a shrink film is required to have both a high shrinkability at about 110 ° C. or higher and a low shrinkage at about 100 ° C. or lower during reheating.

  Also, in pillow packaging, especially when the temperature of the seal bar when sealing the front and back portions of the package is high, the seal portion shrinks in the width direction of the film and may remain as a wrinkle after shrinkage. is there. For this reason, the direction which can be sealed at low temperature is less likely to leave a small portion of the seal portion after shrinkage. However, if the sealing temperature between the films is too low, for example, the film is sealed at a temperature lower than the microwave heating temperature, and there is a problem that the upper and lower films are fused when the package is heated in a range of two layers. .

  Therefore, a shrink film can be sealed at a low temperature, but it is also important that the films do not fuse together during use.

Patent Document 1 discloses a multilayer crosslinked shrink film using an ethylene-α-olefin copolymer having a specific density, and an outer layer resin having an ethylene-α-olefin having an density of 0.913 g / cm ³ and an inner layer. ethylene -α- olefin density density as a resin as a film or layer resin using ethylene -α- olefin 0.915 g / cm ³ is 0.913 g / cm ^3, the density of the inner resin 0.926 g / cm ³ A film using ethylene-α-olefin and high-pressure low-density polyethylene is exemplified.
However, the former has a problem that uneven thickness tends to occur and the shrinkage rate at 120 ° C. is low, and the latter similarly has a low shrinkage rate at 120 ° C., and it is necessary to increase the shrink tunnel temperature during packaging.

  Thus, at present, a shrink film that is highly shrinkable at about 110 ° C. or more, but has low shrinkage at about 100 ° C. or less, and that the films do not melt together during use, and preferably can be sealed at a low temperature. Not obtained.

JP 2002-120343 A

  In particular, the present invention is a multilayer shrink film having excellent extrudability, extrusion stability, stretch stability, high magnification stretchability, tear strength, and uniform thickness, and the resulting film is used to fill the contents. When the container is heated in a microwave oven in a shrink-wrapped state, the container is less deformed and heated in a stacked state when heated in a microwave oven. An object of the present invention is to provide a shrink film having a low shrinkage below the vicinity and a high shrinkage above 110 ° C., and preferably capable of being sealed at a low temperature.

As a result of intensive studies to achieve the above-mentioned problems, the inventor contains an ethylene-α-olefin copolymer having a melt tension at 240 ° C. of 25 mN or more and a melting main peak temperature of 118 ° C. or less in the inner layer. As a result, it was found that high shrinkage near 110 ° C. or higher can be realized while maintaining low shrinkage near 100 ° C. or lower.
That is, the present invention is as follows.
Having at least three layers of both surface layers and an inner layer sandwiched therebetween,
The inner layer has ethylene molecular weight distribution (Mw / Mn) of 4 to 10, density of 0.900 to 0.930 g / cm ³ , melt tension at 240 ° C. of 25 to 100 mN, and melting main peak temperature of 118 ° C. or less. containing 70 to 100% by weight of an α-olefin copolymer, and
The multilayer shrink film whose melt tension in 240 degreeC of the resin composition which comprises an inner layer is 25-100 mN.

  The multilayer shrink film of the present invention achieves both low shrinkage near 100 ° C. or less and high shrinkage near 110 ° C., so that it can be packaged at a low temperature and the film can be reheated during microwave heating or the like. Container deformation caused by shrinkage is small, packaging finish is good, and there is little fusion between films in use.

The present invention will be described in detail below with a focus on preferred embodiments.
1. Surface Layer Material The multilayer shrink film of the present invention is preferably composed of a resin composition in which at least one surface thereof contains an ethylene-α-olefin copolymer having a density of 0.900 to 0.920 g / cm ^3. .
Here, the ethylene-α-olefin copolymer refers to a copolymer of ethylene and at least one monomer selected from α-olefins having 3 to 18 carbon atoms.

  Examples of the α-olefin used as the monomer of the ethylene-α-olefin copolymer include propylene, butene-1, pentene-1, 4-methyl-pentene-1, hexene-1, octene-1, and decene-1. , Dodecene-1 and the like.

  The ethylene content in the copolymer is preferably 40 to 95% by weight, more preferably 50 to 90% by weight, and still more preferably 60 to 85% by weight from the viewpoints of sealing properties and transparency.

  The polymerization catalyst used for producing the copolymer is not particularly limited, and examples thereof include a multi-site catalyst and a single-site catalyst. A single-site catalyst is preferable from the viewpoint of film surface slipperiness.

The density of the ethylene-α-olefin copolymer used for at least one of the surface layers is preferably 0.900 to 0.920 g / cm ³ .
If the density of the ethylene-α-olefin copolymer is 0.900 g / cm ³ or more, the film is stretched and the runnability in the packaging machine is improved, and if it is 0.920 g / cm ³ or less, 110 The shrinkage rate at or above about ° C is further improved.
The density of the ethylene-α-olefin copolymer is more preferably 0.905 to 0.918 g / cm ³ , and still more preferably 0.910 to 0.915 g / cm ³ .

  The molecular weight distribution (Mw / Mn) of the ethylene-α-olefin copolymer used for the surface layer measured by a gel permeation chromatography apparatus (hereinafter referred to as GPC) is such as a stickiness after being formed into a film. From the viewpoint, it is preferably narrower, preferably Mw / Mn is 3.5 or less, more preferably 3.2 or less. On the other hand, from the viewpoint of extrudability, it is preferably 1.5 or more, more preferably 2.0 or more, and further preferably 2.5 or more.

The resin composition containing an ethylene-α-olefin copolymer having a density of 0.900 to 0.920 g / cm ³ contained in at least one of the surface layers is composed of the ethylene-α-olefin copolymer alone. Or a mixture with another polymer. In the case of a mixture with another polymer, the content of the other polymer is 50 with respect to 100 parts by weight of the ethylene-α-olefin copolymer having a density of 0.900 to 0.920 g / cm ^3. The amount is preferably not more than parts by weight, more preferably not more than 30 parts by weight, still more preferably not more than 20 parts by weight.

As another polymer, a high-pressure low-density polyethylene having a density of 0.930 g / cm ³ or less is preferably 1 to 25 parts by weight, more preferably 5 to 20 parts per 100 parts by weight of the ethylene-α-olefin copolymer. Addition of parts by weight is preferable because the hot tack sealability and transparency are improved. Further, as another polymer, an ethylene-α-olefin having a density of 0.850 to 0.900 g / cm ³ is preferably 1 to 25 parts by weight with respect to 100 parts by weight of the ethylene-α-olefin copolymer. More preferably, it is preferable to add 5 to 20 parts by weight because a low temperature seal can be achieved.

The surface layer needs to be able to seal at low temperatures, that is, to be easily melted at low temperatures, in particular from the viewpoint of suppressing small wrinkles around the seal portion, saving power consumption, etc. . Therefore, 2nd. Of the differential scanning calorimeter of the resin composition containing the ethylene-α-olefin copolymer having a density of 0.900 to 0.928 g / cm ³ constituting the surface layer. The ratio of the heat of fusion of 100 ° C. or less to the total heat of fusion in the melting behavior is preferably 40 to 75%. From the viewpoint of low temperature sealing properties, the ratio of the heat of fusion of 100 ° C. or less to the total heat of fusion is preferably 40% or more, more preferably 45% or more, and even more preferably 50% or more. Moreover, from the viewpoint that when a packaging body in which a container with a lid containing lunch boxes, side dishes, or the like is wrapped with the film is heated in a microwave oven, the films are less likely to be fused to each other, preferably 75% or less, and more preferably 65%. % Or less, more preferably 60% or less.

  Furthermore, when the melting characteristics of the surface layer of the present invention are in the above range, the shrinkage balance with the inner layer is good, and the transparency after shrinkage is improved.

  Here, 2nd. Of the differential scanning calorimeter. The ratio of the heat of fusion of 100 ° C. or less to the total heat of fusion in the melting behavior is that the resin composition is heated from 0 ° C. to 200 ° C. at a rate of 10 ° C./min (1st. Melting behavior) and held at 200 ° C. for 1 minute. After that, when the temperature was lowered to 0 ° C. at 10 ° C./min and then again raised to 200 ° C. at 10 ° C./min (2nd. Melting behavior), the total heat of fusion of 20 ° C. to 100 ° C. The value (%) divided by the total heat of fusion in the range until the end of melting.

  The thickness ratio of each surface layer to the total thickness of the multilayer shrink film is preferably 5 to 50%, more preferably 8 to the entire film from the viewpoints of extrudability, sealing properties, transparency, and the like. 30%, more preferably 10 to 20%.

The film of the present invention preferably contains an ethylene-α-olefin copolymer having a density of 0.900 to 0.920 g / cm ³ in at least one of the surface layers. It can be formed using any resin or resin composition as long as the properties of the film of the present invention are not impaired.

Examples of the resin used for the other surface layer include, but are not limited to, ethylene resins and polypropylene resins.
Use of the same material for both surface layers is preferable from the viewpoint of the flatness of the film.

When both surface layers are composed of different materials, either one of both surface layers may be exposed at the time of packaging, but from the viewpoint of film fusion during heating in a microwave oven, the density is 0.900 to 0.920 g / It is preferable that the surface layer containing the cm ³ ethylene-α-olefin copolymer be an exposed surface.

2. Material of inner layer The multilayer shrink film of the present invention has an inner layer having a molecular weight distribution (Mw / Mn) of 4 to 10, a density of 0.900 to 0.930 g / cm ³ , and a melt tension at 240 ° C. of 25 to 25. The melting tension at 240 ° C. of the resin composition comprising 70 to 100% by weight of an ethylene-α-olefin copolymer having a melting main peak temperature of 118 ° C. or lower and constituting the inner layer is 25 to 100 mN. .

  In the present invention, since the ethylene-α-olefin copolymer contained in the inner layer has a wide molecular weight distribution (Mw / Mn) and a high melt tension, a film having excellent stretching stability and a uniform thickness can be obtained. . As a result, the stability of the heat seal is remarkably improved, and the low temperature sealability is ensured.

  From the viewpoint of extrudability, the molecular weight distribution (Mw / Mn) of the ethylene-α-olefin copolymer contained in the inner layer is in the range of 4 to 10, preferably 4.5 to 9, more preferably 5 to 5. 8.

The density of the ethylene-α-olefin copolymer contained in the inner layer is 0.900 to 0.930 g / cm ³ .
If the density of the ethylene-α-olefin copolymer is 0.900 g / cm ³ or more, the film is stretched and the runnability in the packaging machine is improved, and if it is 0.930 g / cm ³ or less, 110 Shrinkage is improved in the vicinity of ℃.

  The melt tension at 240 ° C. of the ethylene-α-olefin copolymer contained in the inner layer needs to be 25 mN or more in order to improve the high shrinkability near 110 ° C. or more. However, from the viewpoint of the extrudability of the unstretched tube or sheet, it is better that the melt tension at 240 ° C. is not too high.

  Therefore, the melt tension of the ethylene-α-olefin copolymer contained in the inner layer is 25 to 100 mN, preferably 28 to 90 mN, more preferably 30 to 80 mN at 240 ° C.

Moreover, it is necessary from the viewpoint of extrudability, extrusion stability, stretch stability, and high magnification stretchability to adjust the melt tension of the resin composition constituting the inner layer to 25 to 100 mN at 240 ° C., preferably Is 27 to 68 mN, more preferably 29 to 66 mN.
By setting it as the above melt tension, extrusion stability improves and the thing of 0-5% can be obtained as a fluctuation range of an unstretched tube.
In the present invention, the “resin composition constituting the layer” refers to a resin composition in which the layer is formed by molding the layer, and when the layer is made of a single resin and does not contain an additive. Such a resin is also included in the resin composition referred to herein.

  In the present invention, the melt tension at 240 ° C. means that a resin or resin composition at 240 ° C. is extruded in a strand form from a capillary having a nozzle diameter of 2.095 mm, and this strand is wound at a winding speed of 1, 3, 5, 7 The maximum tension among the tensions exhibited by the resin or the resin composition when wound at 10, 15, 20, 25, 30 m / min.

In general, ethylene-α-olefin copolymers have low melt tension, so from the viewpoint of extrusion stability of unstretched tubes or sheets, it is considered difficult to use alone as a layer constituent material. It is carried out by adjusting the melt tension of the layer constituent material by mixing with a high-pressure high-density low-density polyethylene or ethylene-vinyl acetate copolymer. However, a resin composition obtained by mixing a high-pressure method low-density polyethylene or an ethylene-vinyl acetate copolymer to increase the melt tension cannot obtain the effect of improving the high shrinkage at around 110 ° C. or higher. Furthermore, since the high pressure method low density polyethylene and the ethylene-vinyl acetate copolymer have lower tear strength than the ethylene-α-olefin copolymer, the higher the blend ratio of the high pressure method low density polyethylene, The tear strength tends to decrease.
In the present invention, by using an ethylene-α-olefin copolymer in which long chain branching is introduced into the copolymer or the melt tension is increased by increasing the degree of polymerization, high shrinkage at about 110 ° C. or higher is achieved. In addition to being able to achieve the improvement of the high pressure method low density polyethylene and ethylene-vinyl acetate copolymer, without being mixed, the extrusion stability and stretch stability of the resin composition constituting the inner layer can be improved, The time and effort of mixing can be saved, and the tear strength of the multilayer film can be improved.

The melting main peak temperature of the ethylene-α-olefin copolymer contained in the inner layer is 118 ° C. or lower, preferably 116 ° C. or lower, more preferably 114 ° C. or lower, from the viewpoint of low temperature shrinkage.
Here, the melting main peak temperature means that the copolymer is heated from 0 ° C. to 10 ° C./min to 200 ° C. (1st. Melting behavior), held at 200 ° C. for 1 minute, and then at 10 ° C./min. It indicates the largest endothermic peak among the endothermic peaks shown when the temperature was lowered to 0 ° C. and then raised again to 200 ° C. at 10 ° C./min (2nd. Melting behavior).

In the present invention, the inner layer contains 70 to 100% by weight of the ethylene-α-olefin copolymer, preferably 75% by weight or more, and more preferably 85% by weight or more. As a result, extrudability, extrusion moldability, and stretching stability can be obtained, and tear strength is improved to reduce troubles such as tear propagation during packaging.
When it is desired to further increase the melt tension and improve the extrusion processability, the material constituting the inner layer may be blended with a high-pressure low-density polyethylene or an ethylene-vinyl acetate copolymer. When blending, the content of the resin other than the ethylene-α-olefin copolymer is 30% by weight or less, preferably 25% by weight or less, and more preferably 15% by weight or less.

  When an ethylene-vinyl acetate copolymer is contained, flexibility and stretch stability can be imparted to the film. Usually, the higher the vinyl acetate content, the lower the melting point and the more flexible, but the preferred vinyl acetate content is 1 to 30% by weight, preferably 1 to 20% by weight, Preferably they are 1 weight% or more and 15 weight% or less.

  The inner layer may contain any other resin such as a polypropylene resin, a styrene resin, or a polybutene resin as long as the transparency of the film is not impaired as long as it is in the range of 0 to 30% by weight. From the viewpoint of compatibility, an ethylene-based resin is preferable.

  Recycled raw materials can be used for the inner layer. The recycled material is a material obtained by pulverizing a film remaining after slitting to a predetermined width, etc., and then extruding it into a pellet. These recycled materials may have a higher molecular weight and a higher melt viscosity due to cross-linking treatment, etc. than non-recycled materials, and if this is used for the surface layer, the film surface will be affected by the difference in viscosity from the non-recycled materials. It may be rough and decrease transparency. However, when used in the inner layer, surface roughness is unlikely to occur and the transparency of the film can be maintained, which is preferable from the viewpoint of resource saving and the like.

3. Multilayer Shrink Film The production method of the multilayer shrink film of the present invention is not particularly limited, and may be produced by forming each layer separately and then bonding them together, but it is preferred to produce by coextrusion by a melt extrusion method. For example, the resin or resin composition constituting each layer can be melted with each extruder and co-extruded with a multilayer circular die or the like.

In the present invention, it is preferable to subject the multilayer shrink film to a cross-linking treatment in order to impart heat resistance and particularly to stretch a thin film of about 10 μm more stably.
Examples of the crosslinking treatment include irradiation with energy rays such as electron beams, ultraviolet rays, X rays, α rays, and γ rays.
The range of the preferable irradiation dose of the crosslinking treatment is 40 to 200 kGy, and 50 to 120 kGy is more preferable from the viewpoint of heat sealability and stretching stability.

A gel fraction is used as a measure of the degree of crosslinking. Here, the gel fraction is the ratio of the portion that remains without being dissolved after the sample is immersed in boiling paraxylene for 12 hours, and is represented by the following equation.
Gel fraction (% by weight) = (sample weight after extraction / sample weight before extraction) × 100

From the viewpoint of heat sealability, the gel fraction of at least one surface layer is preferably 1 to 20% by weight, more preferably 3 to 15% by weight.
When the gel fraction of the surface layer is 1% by weight or more, the transparency of the film is improved, and when it is 20% by weight or less, the heat sealing property is improved, and the sealing property having sufficient strength even when sealed at a low pressure. Is obtained.
The gel fraction is preferably in the range of 5 to 50% by weight and more preferably in the range of 20 to 40% by weight from the viewpoint of stretchability and heat resistance of the film. Moreover, it is preferable that the ratio of the gel fraction of the surface layer used as a heat seal layer and an internal layer is 2: 1 to 1:20 from a viewpoint of making heat seal property and ductility compatible.

The multilayer shrink film of the present invention is preferably stretched. Specifically, the unstretched multilayer shrink film is subjected to a crosslinking treatment, and at least 10 times higher than the melting peak temperature of the resin constituting each layer at least six times in the flow direction and the width direction in at least one direction. It is preferable to perform axial stretching or simultaneous biaxial stretching. In particular, the double bubble inflation method is suitable for stretching a thin film of about 10 μm.
In addition, in this invention, a flow direction means the extrusion direction at the time of film formation, and a width direction means the direction orthogonal to an extrusion direction.

  Specifically, the resin composition constituting each layer is melt-extruded using an extruder and sequentially joined in an annular die one by one, or joined at a time in an annular die to form a multi-layer tubular A stretched fabric is obtained. At this time, one extruder may be used per layer, or it may be divided into two or more by the time the resin composition flows from one extruder into the annular die. . This was rapidly cooled and solidified into the stretching machine, and while the stretching start point was heated to the melting point of the resin composition to the melting point + 40 ° C. or less, air was injected between the nip rolls provided with a speed difference, the flow direction, A method of stretching in the width direction at least 6 times in each direction and at a magnification of 4 to 10 times is preferable. The melting point here is 2nd. Of the differential scanning calorimeter. The peak value at the time of melting in the melting pattern is indicated. When there are two or more peaks, the peak value on the highest temperature side is indicated. By stretching at a temperature equal to or higher than the melting point of the resin composition constituting each layer, high-magnification stretching can be performed and a film having a high maximum shrinkage can be obtained. Moreover, by extending | stretching below melting | fusing point +40 degreeC, it becomes difficult to produce the roughness of a film surface, and transparency and glossiness improve.

The shrinkage of the multilayer shrink film of the present invention is preferably 0% or more and less than 30% at 100 ° C., more preferably 0% to 28%. On the other hand, at 110 ° C., it is preferably 30% or more, more preferably 35% or more, and further preferably 38% or more. Further, at 120 ° C., it is preferably 68% or more, more preferably 70% or more. Furthermore, from the viewpoint of practicality, the shrinkage rate of the multilayer shrink film is preferably 95% or less at 110 ° C. and 120 ° C.
Here, the shrinkage rate refers to a value represented by the following equation, and the shrinkage rate of the film refers to an average value of the shrinkage rate in the film flow direction and the shrinkage rate in the width direction.
Shrinkage rate (%) = {(length before shrinkage−length after shrinkage) / length before shrinkage} × 100

  When the shrinkage rate of the multilayer shrink film is within the above range, it is possible to suppress the deformation of the container at the time of reheating by microwave heating or the like and to achieve low temperature packaging. Low temperature packaging here includes lowering the set temperature of the shrink tunnel, shortening the transit time in the tunnel, suppressing shrinkage wrinkles, lowering the position of the seal line before and after the package, and the like.

Also, when pillow wrapping, if the multilayer shrink film is easy to tear in the longitudinal direction (flow direction), when temporarily wrapping a packaged object with sharp corners with the multilayer shrink film, the film from the contact part with the packaged body May tear and propagate to the vicinity of the feeding portion of the multilayer shrink film. In such a case, it is necessary to start the film through from the beginning, resulting in a large loss.
From such a viewpoint, the tear strength in the longitudinal direction of the multilayer shrink film is preferably 40.0 mN or more, more preferably 60.0 mN or more. On the other hand, from the viewpoint of the unpacking property of the product, the tear strength in the longitudinal direction of the multilayer shrink film is preferably 200.0 mN or less, more preferably 100.0 mN or less.

The layer structure of the multilayer shrink film of the present invention is composed of at least three layers of both surface layers (X and Z) and an inner layer (M). As long as the characteristics of the present invention are not impaired, one or more layers made of any resin such as a polypropylene resin or a styrene resin may be provided as another internal layer (Mn).
The arrangement of the layers when other internal layers are provided is, for example, 4 layers: X / M1 / M / Z, 5 layers: X / M / M1 / M2 / Z, 7 layers: X / M1 / M2 / M / M3 / M4 / Z. In addition, it can be composed of 6 layers, 8 layers, and more layers, and the other inner layers may be arranged at any position between X and Z.

  The obtained multilayer shrink film can be slit into a predetermined size and used for packaging. The thickness of the multilayer shrink film is preferably 5 to 50 μm, more preferably 6 μm or more for tear resistance during packaging, and more preferably 30 μm or less for easy opening after packaging.

  A surfactant or an antifogging agent may be contained in any of the layers constituting the multilayer shrink film of the present invention. For example, a resin composition constituting each layer with at least one additive such as an antifogging agent selected from glycerin fatty acid ester, polyglycerin fatty acid ester, sorbitan fatty acid ester, ethylene oxide adduct and the like, and liquid paraffin as a plasticizer If it is contained in an amount of 0.1 to 10.0% by weight, it is preferable because processability and runnability of the film during packaging are improved. In particular, it is more preferable to add 0.5 to 10% by weight of polyglycerol fatty acid ester or the like from the viewpoint of transparency, and it is more preferable to add 0.8 to 6% by weight from the viewpoint of antistatic properties and slipperiness.

  In any layer constituting the multilayer shrink film of the present invention, natural silica or synthetic silica, saturated fatty acid amide, unsaturated fatty acid amide, talc, etc. may be blended as a lubricant within a range not to impair the original characteristics and transparency. Good.

It is preferable to subject the multilayer shrink film of the present invention to surface treatment such as corona treatment, ozone treatment, or flame treatment because the film is suitable for printing applications.
When performing the printing process, 0.5 to 5.0% by weight of glycerin fatty acid ester or the like is added to the material of the surface layer to be the printing surface, and after the film is formed, the printing surface is subjected to a corona treatment, It is preferable to perform a printing process. From the viewpoint of antistatic properties and prevention of ink peeling, the addition amount of glycerin fatty acid ester and the like is more preferably 0.8 to 3.0% by weight with respect to the resin composition constituting the surface layer.

  Furthermore, in any layer constituting the multilayer shrink film of the present invention, as a plasticizer, a tackifier resin or a petroleum resin (for example, Alcon (trademark), Clearon (registered trademark), Imabe (registered trademark), etc.) If the content is 0.1 to 10% by weight with respect to the resin composition constituting each layer, shrinkage and transparency may be improved.

The present invention will be described based on examples.
The measurement methods used in the examples and comparisons are described below.
(1) Measurement of melt tension A capillary having a length of 8.0 mm and a nozzle diameter of 2.095 mm is attached to the barrel tip of Capillograph 1C (trade name) manufactured by Toyo Seiki Seisakusho, and the barrel temperature is set to 240 ° C. The pellets of the sample were divided into several times, filled in the barrel while sufficiently venting air, and melted. At this time, when the sample was a blend of a plurality of resins, one that was melt-mixed once at a temperature of 200 ° C. was used as a sample pellet. The piston speed is set to 10 mm / min and the resin melted from the capillary is extruded into a strand shape. The strand is hung on a 45 mm diameter tension detection pulley installed 60 cm below the lower surface of the capillary and wound at a constant winding speed. It was. The winding speed is increased in steps of 1, 3, 5, 7, 10, 15, 20, 25, 30 m / min, and the data is captured for 20 seconds when the tension reaches a steady state at each winding speed. The average value of tension was obtained. At each winding speed, the same measurement was performed three times. The average value of n = 3 was defined as the tension at the winding speed, and the maximum tension among the obtained tensions was defined as the melt tension. When the strand was cut before the winding speed reached 30 m / min, the measurement was terminated there, and the maximum tension obtained was taken as the melt tension.

(2) Measurement of extrusion stability An unstretched tube having a thickness of 560 μm extruded from an annular die was water-cooled and solidified, sandwiched by a driven nip roll and flattened, and then its width was measured over 2 m at intervals of 10 cm. The difference between the average value and the maximum value or the minimum value was defined as the fluctuation range.

(3) Evaluation of high magnification stretchability When an unstretched tube is guided into a stretching machine, the temperature at the stretching start point is raised to a temperature 30 ° C. higher than the melting point of the resin, and stretched at a stretch ratio of 7 times in the width direction. The draw ratio in the flow direction was measured.
(4) Measurement of the ratio of the heat of fusion of 100 ° C. or less to the total heat of fusion from Perkin Elmer, Inc., using an input compensated differential scanning calorimeter, Diamond DSC ™, from 0 ° C. to 200 ° C. at 10 ° C./min. The temperature was raised (1st. Melting behavior). After maintaining at 200 ° C. for 1 minute, the temperature was lowered to 0 ° C. at 10 ° C./min (1st. Crystallization behavior). Next, the temperature was raised again to 200 ° C. at 10 ° C./min (2nd. Melting behavior), and the total heat of fusion at 20 ° C. to 100 ° C. at this time was divided by the total heat of fusion in the range from 20 ° C. to the end of melting. Value (%) was adopted. The sample weight was in the range of 5-10 mg.
(5) Measurement of tear strength Set the measurement range of a light load tear tester (trade name) manufactured by Toyo Seiki Co., Ltd. to 50 g, set the film to tear in the flow direction, and calculate the average value measured 10 times. It was set as the tear strength in the film flow direction. The test piece was cut out according to the sampling method described in JIS K7128 with a dimension in the flow direction of 63.5 mm and a width direction of 50 mm.

(6) Measurement of shrinkage rate A 100 mm square film is placed in an air oven type high-temperature bath set at a predetermined temperature (100, 110, 120 ° C.), heat treated for 1 minute, and the film flow direction and width direction at each temperature. The percentages of the values divided by the dimension before shrinkage, that is, 100 mm, were taken as shrinkage rates in the flow direction and width direction, respectively, and the average value of these was taken as the shrinkage rate of the film.

(7) Evaluation of packaging finish The film is slit to a width of 500 mm, and “FW-3451A-αV (trade name)” manufactured by Fujikikai Co., Ltd. is used. Name) ”, about 200 grams of cooked rice at 20 ° C. Each was packed in 30 pillows, heat-treated for 3 seconds in a shrink tunnel set at 130 ° C. and 150 ° C., and the packaging finish was evaluated according to the following criteria: Went.
Fully contracted with no small wrinkles or corner residue around the seal before and after the package: ◎
A small wrinkle around the seal before and after the package remains, but there is no corner residue and it is completely shrunk: ○
A film where the film does not shrink completely and an air pocket remains: ×

(8) Whitening evaluation of high shrinkage part As a visual evaluation, ○ is a high shrinkage part where the corner and the part within 1 cm from the seal line before and after the packaging body are transparent, and x is whitened and opaque. Evaluation of whitening.
(9) Evaluation of heat resistance of microwave oven Two pieces of refrigerated showcases set at 5 ° C in the container of the rice bowl packaged with rice cooked in (7) were stacked for 3 hours to obtain a microwave oven RE-6200A. After heating for 50 seconds (manufactured by Sharp Corporation, rated high-frequency output 1600 W), the film was taken out of the microwave and evaluated for fusion between films. A film that was not fused at all was marked with ◯, and a film that was fused together was marked with x.

(10) Evaluation of container deformation resistance About 200 g of stew at 20 ° C. is put in the same container as in (7), and the product packaged under the same conditions is refrigerated in a refrigerated showcase set at 5 ° C. for 3 hours. After heating at −6200A (manufactured by Sharp Corporation, rated high-frequency output 1600 W) for 30 seconds, the deformation of the container was evaluated. Evaluation was made with a circle indicating no deformation and a mark indicating that there was deformation.

(11) Measurement of gel fraction A sample was extracted in boiling paraxylene for 12 hours, and the ratio of insoluble matter expressed by the following formula was used as a gel fraction and used as a measure of the degree of crosslinking of the film.
Gel fraction (% by weight) = (sample weight after extraction / sample weight before extraction) × 100
(12) Measurement of molecular weight distribution (Mw / Mn) Using GPC (GPC apparatus 150C type (trade name) manufactured by Nippon Waters), the column is TSK GMH-6 (trade name) manufactured by Tosoh, and the solvent is orthodichlorobenzene ( ODCB), and the temperature was 135 ° C., the flow rate was 1 ml / min, the concentration was 10 mg / 10 ml, and the sample flow rate was 500 μl. Mw / Mn was determined from a calibration curve prepared using standard polystyrene.

(13) Evaluation of film thickness uniformity The film thickness was measured at 30 mm intervals over the entire width of the film. The film thickness was read up to 0.5 μm using a dial gauge having a minimum unit of 1 μm. The difference between the maximum value and the minimum value of the film thickness thus measured was defined as the uniformity of the film thickness.

(14) Resins used in Examples and Comparative Examples The following resins were used as materials for the surface layer and the inner layer. Table 1 shows the molecular weight distribution (Mw / Mn), density, melt tension at 240 ° C., and melting main peak temperature of each resin.
LL1: ethylene-α-olefin copolymer (polymerized with a single site catalyst), density = 0.913 g / cm ³ , MI = 2.0 g / 10 min, melting main peak temperature = 113 ° C., Mw /Mn=2.8, Umerit 1520F (trade name) manufactured by Ube Maruzen Polyethylene Co., Ltd.
LL2: ethylene-α-olefin copolymer (polymerized with a multisite catalyst), density = 0.909 g / cm ³ , MI = 1.9 g / 10 min, Exelen VL102 manufactured by Sumitomo Chemical Co., Ltd. (product) Name)
LL3: ethylene-α-olefin copolymer (polymerized with a single site catalyst), density = 0.918 g / cm ³ , MI = 0.9 g / 10 min, Sumikasen EP CU1002 (Sumitomo Chemical Co., Ltd.) Product name)
LL4: ethylene-α-olefin copolymer (polymerized with a single site catalyst), density = 0.912 g / cm ³ , MI = 0.5 g / 10 min, Exelen GMH CB0002 (manufactured by Sumitomo Chemical Co., Ltd.) Product name)
LL5: ethylene-α-olefin copolymer (polymerized with a multi-site catalyst), density = 0.926 g / cm ³ , MI = 2.0 g / 10 minutes, Dow Rex 2032 manufactured by Dow Chemical Co., Ltd. Product name)
LD1: high pressure method low density polyethylene, density = 0.922 g / cm ³ , MI = 0.4 g /
10 minutes, Asahi Kasei Chemicals Corporation Suntech LD M2004

[Examples 1-6, Comparative Examples 1-5]
After extruding a resin composition having the composition shown in Table 2 with 2.0% by weight of diglycerin oleate as a multi-layer (three-layer) raw material from a circular die, it is cooled and solidified with cold water, and a folding width of 120 mm A tube-shaped multilayer original fabric having a thickness of 620 μm was prepared. This was guided to an electron beam irradiation apparatus, irradiated with an electron beam accelerated to 1 MeV, and subjected to a crosslinking treatment so that the absorbed dose was 80 kGy. This is guided into a stretching machine and reheated, passed between two pairs of differential nip rolls, bubbles are formed by air injection, the heating temperature at the stretching start point is set to 140 ° C., and 8 times in the flow direction The film was stretched in the width direction at a magnification of 7 times to obtain a shrink film having an average thickness of 11 μm and a surface layer / inner layer / surface layer thickness ratio (%) of 15/70/15, respectively. In addition, as for the thing which cannot be extended | stretched 8 times in the flow direction, the thickness of the unstretched tube was made thin suitably so that the final average thickness might be 11 micrometers.
The film thus obtained was evaluated for extrusion stability, high-stretchability, shrinkage, tensile strength, and film thickness uniformity.
While cutting both ends of the obtained tube-shaped film, it was cut into a width of 500 mm to obtain two films, each of which was made into one film, having a width of 510 mm, an inner diameter of 76.2 mm, and a thickness of 10 mm. A film for evaluation was prepared by winding the paper roll with a length of 200 m with a tension that does not cause wrinkles. Each of the obtained films for evaluation was packaged with about 200 g of 20 ° C. cooked rice in a PS container of (7), and finished with packaging (such as an air reservoir between the container and the film, a small bowl, a corner residue, etc. (Shrinkability), whitening of the high shrinkage part, microwave oven heat resistance, and container deformation resistance were evaluated.

The results are shown in Table 2.
The films obtained in Examples 1 to 6 each have a difference (R) between the maximum thickness and the minimum thickness of 2.0 μm or less and have excellent tensile strength. There was no corner residue, no wrinkles, and the position of the seal line was lowered, and it was a beautiful package with excellent transparency after shrinkage.
Moreover, there was no fusion | bonding at the time of 2 steps | paragraphs also in microwave oven heating, and the container deformation | transformation was not seen.
In particular, from the comparison between Examples 1, 2, 4 to 6 and Comparative Examples 1 and 2, by increasing the melt tension at 240 ° C. of the ethylene-α-olefin copolymer of the inner layer, at 130 ° C. and 150 ° C. It was confirmed that the high shrinkage of the resin was improved.
Further, from Comparative Example 3, even if the melt tension at 240 ° C. of the entire resin composition is increased by mixing the high-pressure method low density polyethylene, it can be confirmed that the high shrinkage at 130 ° C. and 150 ° C. is not improved. From this, it can be said that the effect of improving the high shrinkage at 130 ° C. and 150 ° C. is due to the increase in the melt tension at 240 ° C. of the ethylene-α-olefin copolymer of the inner layer.

In Comparative Examples 1 and 2, since the melt tension of the inner layer was low, the extrusion stability was poor, and the fluctuation range of the unstretched tube was large. As a result, the difference (R) between the maximum thickness and the minimum thickness of the obtained film was large, and the finish after packaging was insufficient.
In Comparative Example 3, since the ratio of the ethylene-α-olefin copolymer in the inner layer was small, the tear strength of the film in the flow direction of the obtained film was weak, and the melt tension of the inner layer was too high. The high magnification stretchability of the film was inferior, and 5.6 times stretching in the flow direction was the limit. As a result, the shrinkage rate in the flow direction was reduced, wrinkles were likely to remain during packaging, and the packaging finish was inferior.
In Comparative Example 4, since the inner layer does not contain an ethylene-α-olefin copolymer, the tear strength of the film in the flow direction of the obtained film is weak, and the melt tension of the inner layer is too high. The high magnification stretchability was inferior, and 4.8 times stretching in the flow direction was the limit. As a result, the shrinkage rate in the flow direction was reduced, wrinkles were likely to remain during packaging, and the packaging finish was inferior.
In Comparative Example 5, since the melting peak temperature of the ethylene-α-olefin copolymer in the inner layer was high, the shrinkage rate at 110 ° C. of the obtained film was small, and wrinkles remained easily during packaging.

  The multilayer shrink film of the present invention can be suitably used as a packaging film. In particular, it is suitable for packaging containers reheated in a microwave oven, and can be suitably used for packaging lunch boxes, side dishes, and the like.

Claims (5)

Having at least three layers of both surface layers and an inner layer sandwiched therebetween,
The inner layer has ethylene molecular weight distribution (Mw / Mn) of 4 to 10, density of 0.900 to 0.930 g / cm ³ , melt tension at 240 ° C. of 25 to 100 mN, and melting main peak temperature of 118 ° C. or less. containing 70 to 100% by weight of an α-olefin copolymer, and
The multilayer shrink film whose melt tension in 240 degreeC of the resin composition which comprises an inner layer is 25-100 mN. At least one of the surface layers contains an ethylene-α-olefin copolymer having a density of 0.900 to 0.920 g / cm ³ , and the 2nd. The multilayer shrink film of Claim 1 comprised from the resin composition whose ratio of the heat of fusion below 100 degreeC with respect to the total heat of fusion in a melting behavior is 40 to 75%.   The film shrinkage rate at 100 ° C is 0% or more and less than 30%, the film shrinkage rate at 110 ° C is 30% or more, and the film shrinkage rate at 120 ° C is 68% or more. A multilayer shrink film as described in 1.   The multilayer shrink film according to any one of claims 1 to 3, wherein a molecular weight distribution (Mw / Mn) of the ethylene-α-olefin copolymer contained in the surface layer is 3.5 or less.   The multilayer shrink film according to any one of claims 1 to 4, wherein a melt tension at 240 ° C of the resin composition constituting the inner layer is 30 to 70 mN.